The answer to the question, 'What should I believe, and why should I believe it?' is generally a scientific one. Believe a proposition because it is well supported by theory and evidence; believe it because it has been experimentally verified; believe it because a generation of smart people have tried their best to falsify it and failed; believe it because it is true (or seems so). This is a norm of cognition as well as the core of any scientific mission statement. As far as our understanding of the world is concerned--there are no facts without values.
Sam Harris The Moral Landscape: How Science Can Determine Human Values